Synapses on CA1 hippocampal neurons have become a leading model system for understanding CNS synapses and the activity-dependent functional changes that are thought to underlie memory. Structural studies of these synapses have revealed a remarkable diversity of synapse size and diversity in the structure and size of the dendritic spines on which the synapses occur. The function of this diversity is unknown. CNS synapses have other structural specializations, the function of which is also unknown. The general goal of this project is to study the function of single identified synapses and to relate their function to their structure. Critical to this goal is the newly developed method of optical quantal analysis. The method is based on the finding that active synapses can be identified by Ca2+ signals in the spine head using a high-speed confocal microscope. These signals, in conjunction with whole-cell recording, make it possible to characterize an identified synapse by determining the quantal-analysis parameters, p and q. 3D EM reconstructions will be made of the same synapses that are physiologically characterized. Ultrastructural features, such as the size and type of active zone, will be determined. By comparing structure to function, it will be possible to determine whether size is the primary determinant of synaptic strength or whether strong modulatory processes are also at work. Optical quantal analysis will also be used to study the effect of LTP at individual synapses. The data obtained may resolve the controversy regarding the presynaptic/postsynaptic locus of LTP expression. The ability to monitor the function of individual CNS synapses and the ability to relate their function to their structure should have wide ranging applications and wide ranging implications for understanding the synaptic malfunctions that underlie disorders of memory.